In U.S. Pat. No. 5,122,240 (E437), assigned to the assignee hereof, there is described the electrochemical acidification of aqueous solutions of alkali metal salt solutions, particularly sodium sulfate, sodium chlorate and mixtures thereof, using an electrolytic cell having a cation-exchange membrane separating the anode and cathode compartments. The resulting acid solution is used to provide acid to a chlorine dioxide generator. The acidity level which can be achieved while retaining an acceptable level of current efficiency (greater than 70%), is quite limited in the case of sodium chlorate alone, although higher levels can be achieved with sodium sulfate and particularly mixtures of sodium sulfate and sodium chlorate.
It has also been described in U.S. Pat. No. 4,915,927 (E433), assigned to the assignee hereof, how to produce chloric acid electrochemically from sodium chlorate, using a three-compartment cell, in which sodium chlorate is fed to the central compartment, chlorate ions migrate through an anion-exchange membrane to an anode compartment wherein hydrogen ions are produced from an anolyte and the sodium ion migrate through a cation-exchange membrane to a cathode compartment wherein hydroxyl ions are produced from a catholyte. Chloric acid is removed from the anode compartment while sodium hydroxide is removed from the cathode compartment.
In U.S. Pat. No. 5,084,148, assigned to Olin Corporation, there is described the production of mixtures of chloric acid and sodium chlorate again in a three-compartment cell, but in this instance, two cation-exchange membranes are used and the mixture of chloric acid and sodium chlorate is removed from the central compartment.
In another embodiment of U.S. Pat. No. 5,084,148, the production of a mixture of chloric acid and sodium chlorate is effected in a four-compartment cell, equipped with three cation-exchange membranes wherein sodium chlorate solution is fed to the compartment adjacent to the cathode compartment and the product then is fed to the compartment adjacent to the anode compartment of the same cell where it is acidified further. A non-oxidizable acid, such as sulfuric acid, is circulated in the anode compartment.
As in the case of U.S. Pat. No. 5,122,240 mentioned above, the acidity level of chloric acid which can be achieved using these procedures at a reasonable level of current efficiency is quite low.